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Topic 4 : I/O Devices & Networks I/O Devices and Techniques Reference : G & L pp 169-170 (note that most of the material isnt in G & L)

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Presentation on theme: "Topic 4 : I/O Devices & Networks I/O Devices and Techniques Reference : G & L pp 169-170 (note that most of the material isnt in G & L)"— Presentation transcript:

1 Topic 4 : I/O Devices & Networks I/O Devices and Techniques Reference : G & L pp (note that most of the material isnt in G & L)

2 I/O Devices Examples of input devices: keyboard optical character reader microphone Examples of output devices: video screen printer loudspeaker

3 I/O Devices Some devices can be used for both input and output: magnetic disks magnetic tapes other computers Devices which store large amounts of information such as tapes and disks are called secondary storage.

4 Hardware Device Differences Speed Locating data Transferring data Unit of transfer character block Permissible operations whether or not device is read only whether device supports random or sequential access

5 Magnetic Disks A magnetic disk consists of one or more circular metal plates coated with a material which can be magnetised. There may be more than one of these plates mounted on a common spindle.

6 Magnetic Disks The disks rotate continuously. Information is stored on the disk using a set of read/write heads.

7 Tolerances and Head Crashes The disks rotate at high speed: 3,600 – 10,000 rpm. To avoid wear on the disk the heads float above the surface of the disk in the air currents close to the surface. If the surface is contaminated the heads crash and the disk is damaged hence disks are normally sealed.

8 So Just How Close Is Close ? Disk Surface Heads 0.2 µm

9 So Just How Close Is Close ? Disk Surface Heads Smoke 0.2 µm 0.6 µm

10 So Just How Close Is Close ? Disk Surface Heads Smoke Human Hair 0.2 µm 0.6 µm 70 µm

11 Data Layout on Magnetic Disks As the disk rotates, the read/write heads trace out a circular path or track on each surface. On disks with more than one plate, the heads trace out a cylinder.

12 Data Layout on Magnetic Disks Each track is divided up into a fixed number of sectors. Each sector holds a fixed number of bytes of information.

13 Data Layout on Magnetic Disks Each track is divided up into a fixed number of sectors. Each sector holds a fixed number of bytes of information.

14 Data Layout on Magnetic Disks Each track is divided up into a fixed number of sectors. Each sector holds a fixed number of bytes of information. The specification of how many tracks, how many sectors, sector size etc. is called the disk format.

15 Reading and Writing Data Smallest unit which may be read/written is the sector. Two sources of delay before a specified sector can be accessed and transfer commence rotational delay (or latency) seek time (time for r/w head to move radially) Measure these as averages… Average rotational delay (or latency) Proportional to the drives rpm Average seek time (also called average access time) Time (on average) to move to a particular cylinder

16 Optical Storage: CD-ROMs CD-ROM has emerged as a key component in modern computer systems. Conventional CD-ROMs are read-only (as with audio CDs). CD-ROMs can store approx 660 Mbytes (4 cds for Office 2000!). They are cheap to produce, easy to transport and relatively error-free.

17 Optical Storage: CD-ROMs CD-ROMs read data using a laser which scans the surface of the disk. The laser doesnt touch the surface of the disk so there is no wear. Constant Linear Velocity

18 Optical Storage: CD-ROMs CD-ROMs contain tracks which may be either data, audio or video tracks (disks which contain both types of tracks are called mixed-mode disks). Also write once (CD-R) and write many (CD-RW) Comparatively slow access times (cf HD) Mass of head-assembly Data track on a CD is one long, continuous spiral The track advances outward from centre of disk CD-ROMs have sectors (large frames)…

19 Sector Structure in CD-ROMs CD-ROMs contain 333,000 sectors 74 minutes of audio Frames are read at 75 sectors per second (single speed). Each block contains 2352 bytes: 12 bytes to mark the start of the block 4 bytes for a header 2048 bytes of user data 4 bytes for error detection 8 unused bytes 276 bytes for error correction

20 CD-ROM Capacity Calculations Capacity is calculated as follows: 333,000 blocks * 2048 bytes per block = 681,984,000 bytes 681,984, Mbytes Data rate is: 2048 bytes * 75 blocks per second 150 Kbytes/sec (1x) 40x 6000 Kbytes/sec or 6 Mbytes/sec (maximum transfer)

21 The High Sierra Format Defines how to organise data into files Worked out by a group of industry representatives at the High Sierra Hotel and Casino in Nevada. Basis for ISO 9660 standard. Describes the logical data format for CD-ROMs, e.g. the directory structure the table of contents

22 Standards for CD-ROM Many industry standards to meet different application requirements: Red book:describes CD-DA (Digital Audio) Yellow book:describes mixed media format Green book:governs CD-i Orange book:standard for write-once CDs Blue book:stamped multi-session CDs (CD- Plus) White book:Video-CD (based on CD-i)

23 DVD – Digital Versatile Disk Digital Versatile Disk Closely resembles CD-ROM technology Information as pits, arranged along concentric, circular tracks embedded in reflective material But, laser has shorter wavelength, so can detect smaller pits, giving more pits per track and so more tracks per disk. Can read standard CD-ROMs. DVD-ROM, movies etc. DVD-RAM, high-capacity read/write storage.

24 DVD Formats Different formats… Single-sided, single-layer4.7 Gb Double-sided, single-layer9.4 Gb Storage for approx two feature length films. Single-sided, double-layer8.5 Gb Effectively two disks bonded together Upper disk has partially transmissive surface Lower disk has fully reflective surface Laser can focus on either surface Double-sided, double-layer17 Gb

25 Other Peripherals

26 A Common Input Device: The Mouse Mouse was devised by Prof. Englebart of Stanford University. All mice are either: mechanical, opto-mechanical, or, optical. The difference depends on how they detect position.

27 Mechanical Mice Movement is detected using electrical contacts on disks rotated by movement of the ball.

28 Opto-Electrical Mice Movement is detected using photo-cells and LEDs instead of contacts.

29 Optical Mice Optical mice detect movement by sensing the patterns of light reflected from a special mouse mat.

30 Output Devices: Printers Printers can be classified as either: Impact, or, Non-impact. Examples of impact printers are: Dot matrix printers Daisy wheel printers Examples of non-impact printers are: Ink jets Laser printers

31 Ink Jet Printers Non-impact technology: characters are created by squirting ink at the page. Similar mechanism to dot matrix printer

32 Inkjet printers continued… Three types of inkjet engines– all based on different approaches to breaking surface tension of ink to allow it to squirt through small nozzle Thermal inkjets Heat used to burst an ink bubble (e.g. Canon Bubble Jet) Piezo inkjets Squirt gun approach (e.g. Epson Stylus printers) Phase change inkjets. Ink changes phase from solid to liquid (solid inkjet printer) Inkjets have numerous advantages over dot matrix printers.

33 Laser Printers Non-impact technology: laser printers work in a very similar fashion to photocopies. A drum is charged. The image is then projected onto the drum in such a way that different parts of the drum have different charges. Toner is charged and attracted to the drum. The drum is rolled against the paper and the toner set using heat.

34 Laser Printers

35 Printer Selection Printer language. E.g. Postscript, PCL Quality. DPI Images. Bitmap (array of dots) or vector (set of lines or curves) Speed. ppm Other factors Noise, Reliability, Paper handling, simplex/duplex

36 Summary Listed examples of I/O devices. Looked at the construction of: Magnetic Disks CD-ROMs Mice Printers

37 Coming Next Week Interfacing to I/O devices, DMA etc.. G & L pp


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